Supply-switched dual cell memory bitcell

ABSTRACT

In one embodiment, a bit state in a supply-switched dual cell memory bitcell in accordance with the present description, may be read by coupling a supply line to a common node of the bitcell to drive complementary currents through complementary resistance state storage cells for a pair of complementary bit line signal lines of the bitcell. The bit state of the bitcell may be read by sensing complementary bit state signals on the pair of first and second complementary bit line signal lines. In one embodiment, each resistance state storage cell has a resistance state ferromagnetic device such as a magnetic-tunneling junction (MTJ). In one embodiment, a supply-switched dual cell memory bitcell in accordance with the present description may lack a source or select line (SL) signal line. Other aspects are described herein.

TECHNICAL FIELD

Certain embodiments of the present description relate generally tonon-volatile memory.

BACKGROUND

Spin Transfer Torque Random Access Memory (STTRAM) is a type ofmagnetoresistive Random Access Memory (MRAM) which is non-volatile andis typically used for memory circuits, such as, cache, memory, secondarystorage, and other memory applications. STTRAM memory may often beoperated at reduced power levels and may be less expensive as comparedto other memory types.

Spin Transfer Torque (STT) is an effect in which the orientation of amagnetic layer in a magnetic tunnel junction (MTJ) device can bemodified using a spin-polarized current. In STT-based MTJs, deviceresistance can be either low or high, depending on the relative angulardifference between the directions of magnetic polarization on both sidesof the tunnel junction. Accordingly, one bit state of an MTJ may berepresented by a state in which the ferromagnetic layers of the MTJ havea parallel magnetic orientation and exhibit low resistance. Conversely,an opposite bit state may be represented by a state in which theferromagnetic layers of the MTJ have an anti-parallel magneticorientation and exhibit high resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present disclosure are illustrated by way of example,and not by way of limitation, in the figures of the accompanyingdrawings in which like reference numerals refer to similar elements.

FIG. 1 depicts a high-level block diagram illustrating one embodiment ofa system employing a supply-switched dual cell memory bitcell inaccordance with the present description.

FIG. 2 depicts a basic architecture of an STTRAM memory in accordancewith an embodiment of the present disclosure.

FIGS. 3a-3d depict an embodiment of a supply-switched dual cell memorybitcell in accordance with the present description.

FIGS. 4a, 4b . depict various polarizations of ferromagnetic layers of aferromagnetic device of a bitcell of the STTRAM memory of FIG. 2.

FIG. 5a depicts an embodiment of read operations in a bitcell of theSTTRAM memory of FIG. 2.

FIG. 5b depicts an embodiment of write operations in a bitcell of theSTTRAM memory of FIG. 2.

FIG. 6 depicts an example of a prior art bitcell of an STTRAM memory.

FIG. 7 depicts another example of a prior art bitcell of an STTRAMmemory.

FIGS. 8a-8c depict another example of a prior art bitcell of an STTRAMmemory.

FIG. 9 depicts a prior art array of the prior art bitcells of FIGS. 8a-8 c.

FIG. 10 depicts an array of bitcells of FIGS. 3a-3d in accordance withan embodiment of the present disclosure.

FIGS. 11a-11d depict another embodiment of a supply-switched dual cellmemory bitcell in accordance with the present description.

FIG. 12 depicts yet another embodiment of a supply-switched dual cellmemory bitcell in accordance with the present description.

DESCRIPTION OF EMBODIMENTS

In the description that follows, like components have been given thesame reference numerals, regardless of whether they are shown indifferent embodiments. To illustrate one or more embodiments of thepresent disclosure in a clear and concise manner, the drawings may notnecessarily be to scale and certain features may be shown in somewhatschematic form. Features that are described or illustrated with respectto one embodiment may be used in the same way or in a similar way in oneor more other embodiments or in combination with or instead of featuresof other embodiments.

In one aspect of the present description, a supply-switched dual cellmemory bitcell has a reduced number of signal lines for storing andreading a data bit. For example, in one embodiment, the supply-switcheddual cell memory bitcell lacks a source or select line (SL) signal linetypical of many previous existing and proposed STTRAM bitcells. Asexplained in greater detail below, by eliminating the SL signal line,layout restrictions of circuit elements may be relaxed, which mayfacilitate cell size shrinkage as circuit fabrication processes advance.In addition, a supply line to the bitcell may be meshed in the form ofan orthogonal grid of supply lines to provide increased reliability andstability of bitcell operations such as read operations.

In one embodiment, a bit state in a supply-switched dual cell memorybitcell in accordance with the present description, may be read bycoupling a supply line such as power supply line or ground supply line,for example, to a common node of the bitcell to drive first and secondcomplementary currents through first and second complementary resistancestate storage cells, respectively, of the bitcell, for a pair of firstand second complementary bit line signal lines, respectively, of thebitcell. The bit state of the bitcell may be read by sensingcomplementary bit state signals on the pair of first and secondcomplementary bit line signal lines.

In one embodiment, each resistance state storage cell has a resistancestate ferromagnetic device such as a magnetic-tunneling junction (MTJ).Each resistance state ferromagnetic device has a resistance statecomplementary to a resistance state of the resistance stateferromagnetic device of the complementary resistance state storage cell.

Read operation controller logic of a memory controller is configured toread the bit state of the bitcell by activating first and second storagecell switch transistors of the first and second complementary resistancestate storage cells, respectively, to permit the first and secondcomplementary currents to pass through the first and secondcomplementary resistance state storage cells. In addition, a supplyswitch transistor of the bitcell is activated to couple the supply lineto the common node of the bitcell to drive the first and secondcomplementary currents through the first and second complementaryresistance state storage cells, respectively. In this manner, thebitcell provides complementary bit state signals on the complementarybit line signal lines wherein the bit state of the bitcell is read.Other aspects and features of a supply-switched dual cell memory bitcellin accordance with the present description, are described below.

Although described in connection with an STTRAM, it is appreciated thata supply-switched dual cell memory bitcell in accordance with thepresent description may be applied to MRAM devices other than STT MRAMdevices such as giant magnetoresistance (GMR) MRAM, toggle MRAM andother MRAM devices. Such MRAM-based memory elements in accordance withembodiments described herein can be used either in stand-alone memorycircuits or logic arrays, or can be embedded in microprocessors and/ordigital signal processors (DSPs). Additionally, it is noted thatalthough systems and processes are described herein primarily withreference to microprocessor based systems in the illustrative examples,it will be appreciated that in view of the disclosure herein, certainaspects, architectures, and principles of the disclosure are equallyapplicable to other types of device memory and logic devices.

Turning to the figures, FIG. 1 is a high-level block diagramillustrating selected aspects of a computing system implemented,according to an embodiment of the present disclosure. System 10 mayrepresent any of a number of electronic or other computing devices, thatmay include a memory device. Such electronic devices may includecomputing devices such as a mainframe, server, personal computer,workstation, telephony device, network appliance, virtualization device,storage controller, portable or mobile devices (e.g., laptops, netbooks,tablet computers, personal digital assistant (PDAs), portable mediaplayers, portable gaming devices, digital cameras, mobile phones,smartphones, feature phones, etc.) or component (e.g. system on a chip,processor, bridge, memory controller, memory, etc.). System 10 can bepowered by a battery, renewable power source (e.g., solar panel),wireless charging, or by use of an AC outlet.

In alternative embodiments, system 10 may include more elements, fewerelements, and/or different elements. Moreover, although system 10 may bedepicted as comprising separate elements, it will be appreciated thatsuch elements may be integrated on to one platform, such as systems on achip (SoCs). In the illustrative example, system 10 comprises amicroprocessor 20, a memory controller 30, a memory 40 and peripheralcomponents 50 which may include, for example, video controller, inputdevice, output device, storage, network adapter, a power source(including a battery, renewable power source (e.g., photovoltaic panel),wireless charging, or coupling to an AC outlet), etc. The microprocessor20 includes a cache 25 that may be part of a memory hierarchy to storeinstructions and data, and the system memory 40 may also be part of thememory hierarchy. Communication between the microprocessor 20 and thememory 40 may be facilitated by the memory controller (or chipset) 30,which may also facilitate in communicating with the peripheralcomponents 50.

Storage of the peripheral components 50 may be, for example,non-volatile storage, such as solid-state drives, magnetic disk drives,optical disk drives, a tape drive, flash memory, etc. The storage maycomprise an internal storage device or an attached or network accessiblestorage. The microprocessor 20 is configured to write data in and readdata from the memory 40. Programs in the storage are loaded into thememory and executed by the processor. A network controller or adapterenables communication with a network, such as an Ethernet, a FiberChannel Arbitrated Loop, etc. Further, the architecture may, in certainembodiments, include a video controller configured to displayinformation represented by data in a memory on a display monitor, wherethe video controller may be embodied on a video card or integrated onintegrated circuit components mounted on a motherboard or othersubstrate. An input device is used to provide user input to theprocessor, and may include a keyboard, mouse, pen-stylus, microphone,touch sensitive display screen, input pins, sockets, or any otheractivation or input mechanism known in the art. An output device iscapable of rendering information transmitted from the processor, orother component, such as a display monitor, printer, storage, outputpins, sockets, etc. The network adapter may embodied on a network card,such as a Peripheral Component Interconnect (PCI) card, PCI-express, orsome other I/O card, or on integrated circuit components mounted on amotherboard or other substrate.

One or more of the components of the device 10 may be omitted, dependingupon the particular application. For example, a network router may lacka video controller, for example.

Any one or more of the memory devices 25, 40, and the other devices 10,30, 50 may include a supply-switched dual cell memory bitcell inaccordance with the present description, or be embodied as any type ofdata storage capable of storing data in a persistent manner (even ifpower is interrupted to non-volatile memory) such as but not limited toany combination of memory devices that use chalcogenide phase changematerial (e.g., chalcogenide glass), three-dimensional (3D) crosspointmemory, or other types of byte-addressable, write-in-place non-volatilememory, ferroelectric transistor random-access memory (FeTRAM),nanowire-based non-volatile memory, phase change memory (PCM), memorythat incorporates memristor technology, Magnetoresistive random-accessmemory (MRAM) or another Spin Transfer Torque (STT)-MRAM.

FIG. 2 shows an example of a rectangular or orthogonal array 60 of rowsand columns of MRAM bitcells such as the bitcells 64 of an STT cachememory 66 in accordance with one embodiment of the present description.Each bitcell 64 is configured to store a bit state.

The STT cache memory 66 may also include a row decoder, a timer deviceand I/O devices (or I/O outputs). Bits of the same memory word may beseparated from each other for efficient I/O design. A multiplexer (MUX)may be used to connect each column to the required circuitry during aREAD operation. Another MUX may be used to connect each column to awrite driver during a WRITE operation. A control circuit 67 such as amemory controller is configured to control and perform read and writeoperations directed to the bitcells 64 as explained below. The controlcircuit 67 is configured to perform the described operations usingappropriate hardware, software or firmware, or various combinationsthereof.

In one embodiment, each bitcell 64 of the array 60 of bitcells 64includes a complementary pair of resistance state storage cells 68 a, 68b (FIG. 3a-3d ), each of which includes a storage cell switch transistor69 a, 69 b and a resistance state ferromagnetic device 70 a, 70 b suchas a spin valve, or a magnetic tunnel junction (MTJ) device as shown inFIG. 3a . In addition, a supply switch transistor 71 is configured toselectively couple when activated, a Vc power supply line such as aconstant voltage or constant current line 210 to the dual cells 68 a, 68b for read operations as explained in greater detail below. Althoughdepicted as a voltage or current power supply line, it is appreciatedthat the supply line 210 may be a ground (GND) supply line, dependingupon the particular application.

In the illustrated embodiment, the switch transistors 69 a, 69 b, 71 maybe n-channel metal-oxide semiconductor (NMOS) transistors. Thus, in thisexample, the supply switch transistor 71 is controlled by a true drivesignal such word line read (WLR) signal, for example. However, it isappreciated that the switch transistors such as the supply switchtransistor 71 may be p-channel (PMOS) transistor and controlled by acomplementary drive signal such as /WLR, for example, in otherembodiments. Although depicted as metal-oxide semiconductor (MOS)transistors, it is appreciated that the switch transistors such as theswitch transistors 69 a, 69 b, 71 may be other types of switches andother types of transistors such as bipolar (including npn and pnp) forexample.

In one aspect of the present description, the supply-switched dual cellmemory bitcell 64 of FIGS. 3a-3d has a reduced number of signal linesfor storing and reading a data bit. In the embodiment of FIGS. 3a-3d ,the bitcell 64 lacks a source-line or select line (SL) signal linetypical of many previous existing and proposed bitcells. As explained ingreater detail below, by eliminating the SL signal line, the layoutrestrictions of circuit elements may be relaxed, which may facilitatecell size shrinkage as circuit fabrication processes advance. Inaddition, the Vc power or ground supply line 210 may be a meshed in theform of a grid to provide increased reliability and stability of bitcelloperations such as read operations may be enhanced as well.

In this embodiment, the resistance state ferromagnetic device 70 a of abitcell comprises two layers, a free layer 72 a, and a fixed layer 74 a(FIG. 4a ) of ferromagnetic material separated by an intermediate layer76 which is a metallic layer in the case of a spin valve or is a thindielectric or insulating layer in the case of an MTJ. In this example,the layer 72 a of ferromagnetic material is contacted by an electricalcontact layer 78 and has a first polarization in which the magnetizationdirection which predominates is in a first but changeable direction.Because the magnetization direction of the layer 72 a is changeable, itis referred to as the free layer. In the embodiment of FIG. 4a , thepredominant magnetization direction of the free layer 72 a of theresistance state ferromagnetic device 70 a has a magnetization directionrepresented by an arrow 80 a pointing from right to left in thecross-sectional view of FIG. 4 a.

Similarly, the free layer 72 b of ferromagnetic material is contacted byan electrical contact layer 78 of the complementary resistance stateferromagnetic device 70 b and also has a changeable polarization inwhich the magnetization direction which predominates, can be caused tobe in the same or opposite direction as that of the free layer 72 a ofthe resistance state ferromagnetic device 70 a, or can be caused to bein the opposite direction to that of the free layer 72 a of theresistance state ferromagnetic device 70 a as shown in FIGS. 4a, 4b ,for example. Thus, the predominant magnetization direction of the freelayer 72 b of the resistance state ferromagnetic device 70 b has amagnetization direction represented by an arrow 80 b pointing from rightto left in the cross-sectional view of FIG. 4 b.

The other layer 74 a, 74 b of ferromagnetic material of each resistancestate ferromagnetic device 70 a, 70 b is contacted by an electricalcontact layer 81 and is referred to as the “fixed layer” which has anon-changeable polarization in which the predominant magnetizationdirection of the fixed layer may not be selectively changed. Thepredominant magnetization direction of the fixed layer 74 a of theresistance state ferromagnetic device 70 a is represented by an arrow 82a which also points from right to left in the cross-sectional view ofFIG. 4a . Similarly, the predominant magnetization direction of thefixed layer 74 b of the resistance state ferromagnetic device 70 b isrepresented by an arrow 82 b which also points from right to left in thecross-sectional view of FIG. 4 b.

In the example of FIG. 4a , the predominant magnetization directions ofboth the fixed and free layers 74 a, 72 a of the resistance stateferromagnetic device 70 a are depicted as being the same, that is in thesame direction. If the predominant magnetization directions of the twoferromagnetic layers 72 a, 74 a are the same, the polarizations of thetwo layers are referred to as being “parallel.” In the parallelpolarization, the bitcell exhibits a low resistance state which may beselected to represent one of a logical one or a logical zero bit statestored in the bitcell.

Conversely, in the example of FIG. 4b , the predominant magnetizationdirections of both the fixed and free layers 74 b, 72 b of thecomplementary resistance state ferromagnetic device 70 b are depicted asbeing opposite, that is in opposite directions. If the predominantmagnetization directions of the two ferromagnetic layers are opposite,the polarizations of the two layers 72 b, 74 b are referred to as being“anti-parallel.” In the anti-parallel polarization, the bitcell exhibitsa high resistance state which may be selected to represent the other oneof a logical one or a logical zero bit state stored in the bitcell.

In the embodiment of FIGS. 4a, 4b , the magnetization directions of thefixed and free layers are depicted as being generally parallel to thecontact layers 78, 81. However, it is appreciated that in otherembodiments, the magnetization directions of the fixed and free layersmay be oriented in other directions such as being generally orthogonal,for example, to the contact layers 78, 81.

FIG. 5a depicts one example of bit state read operations of thesupply-switched dual cell memory bitcell 64 (FIG. 4) by the memorycontrol circuit 67 (FIG. 2). In one operation, a supply line such as Vcpower supply line 210 (FIG. 3b ) is coupled (block 214, FIG. 5a ) to acommon node 220 (FIG. 3b ) of the bitcell 64 by activation of the supplyswitch transistor 71 which is configured to couple the common node 220to the supply when activated. In one embodiment, the supply switchtransistor 71 is activated by read operation controller logic 212 (FIG.2) of the memory control circuit 67 which is configured to activate thesupply switch transistor 71 by driving a word line read (WLR) signalline 228 a (FIG. 3b ) coupled to an input 229 of the supply switchtransistor 71, to a true state, which turns the supply switch transistor71 to an ON state, permitting a constant power current I_power to flowfrom the Vc power supply line 210 through the supply switch transistor71 to the common node 220.

In response, first and second complementary read currents, I_read1,I_read2, (I_read1+I_read2=I_power), are driven (block 232, FIG. 5a )from the common node 220, through first and second complementaryresistance state storage cells, 68 a, 68 b, respectively, to a pair offirst and second complementary bit line BL, /BL signal lines 230 a, 230b, respectively, of the bitcell 64. In one embodiment, activation ofstorage cell switch transistor 69 a of the first resistance statestorage cell 68 a permits the read current I_read1 to pass through theseries-connected MTJ 70 a and the storage cell switch transistor 69 a ofthe resistance state storage cell 68 a, to the bit line BL signal line230 a of the bitcell 64. Thus, the resistance state storage cell 68 ahas an input coupled to the common node 220 and an output coupled to thebit line BL signal line 230 a of the bitcell 64. More specifically, theseries-connected MTJ 70 a and the storage cell switch transistor 69 a ofthe resistance state storage cell 68 a are connected between the commonnode and 220 and the bit line BL signal line 230 a of the bitcell 64.

In one embodiment, the storage cell switch transistor 69 a is activatedby read operation controller logic 212 (FIG. 2) of the memory controlcircuit 67, which is configured to drive a word line WL signal line 228b coupled to an input 231 a of the storage cell switch transistor 69 a,to a true state, which turns the storage cell switch transistor 69 a toan ON state, permitting read current I_read1 to flow through theresistance state ferromagnetic device (MTJ) 70 a and the storage cellswitch transistor 69 a to the bit line BL signal line 230 a. The readcurrent I_read1 on the bit line BL signal line 230 a provides a bitstate signal on the bit line BL signal line 230 a. Thus, in theembodiment of FIGS. 3a, 3b , the MTJ 70 a is coupled at one end to thecommon node 220 and coupled at the other end to one end of the storagecell switch transistor 69 a, which is coupled at its other end to thebit line BL signal line 230 a of the bitcell 64.

Similarly, activation of storage cell switch transistor 69 b of thefirst resistance state storage cell permits the complementary readcurrent I_read2 to pass through the series-connected MTJ 70 b and thestorage cell switch transistor 69 b of the complementary resistancestate storage cell 68 b, to the complementary bit line /BL signal line230 b of the bitcell 64. Thus, the resistance state storage cell 68 bhas an input coupled to the common node 220 and an output coupled to thebit line /BL signal line 230 b of the bitcell 64. More specifically, theseries-connected MTJ 70 b and the storage cell switch transistor 69 b ofthe resistance state storage cell 68 b are connected between the commonnode and 220 and the bit line /BL signal line 230 b of the bitcell 64.

In one embodiment, the storage cell switch transistor 69 b is activatedby read operation controller logic 212 (FIG. 2) of the memory controlcircuit 67, which is configured to drive the word line WL signal line228 b coupled to an input 231 b of the storage cell switch transistor 69b, to a true state, which turns the storage cell switch transistor 69 bto an ON state, permitting read current I_read2 to flow through theresistance state ferromagnetic device (MTJ) 70 b and the storage cellswitch transistor 69 b to the bit line /BL signal line 230 b. The readcurrent I_read2 on the complementary bit line /BL signal line 230 bprovides a complementary state signal on the complementary bit line /BLsignal line 230 b. Thus, in the embodiment of FIGS. 3a-3d , the MTJ 70 bis coupled at one end to the common node 220 and coupled at the otherend to one end of the storage cell switch transistor 69 b coupled at itsother end to the complementary bit line /BL signal line 230 b of thebitcell 64.

The complementary bit state signals represented by the complementaryread currents I_read1, I_read2, on the complementary bit line BL, /BLsignal lines, 230 a, 230 b, respectively are sensed, that is read (block240, FIG. 5a ) by a suitable sense amplifier. If the resistance stateferromagnetic device 70 a has, for example, a parallel polarization suchthat the resistance state ferromagnetic device 70 a exhibits arelatively low resistance (compared to that of the complementaryresistance state ferromagnetic device 70 b), and the complementaryresistance state ferromagnetic device 70 b has the complementaryanti-parallel polarization such that the resistance state ferromagneticdevice 70 b exhibits a relatively high resistance (compared to that ofthe resistance state ferromagnetic device 70 a), the current I_read1 onthe bit line BL signal line 230 a will be sensed as relatively high(compared to that of the complementary current I_read2), and the currentI_read2 on the complementary bit line /BL signal line 230 b will besensed as relatively low (compared to that of the current I_read1), andthe bit state of the bitcell 64 will be sensed, that is read, asrepresenting a logical one bit state if the low resistance state of theresistance state ferromagnetic device 70 a is selected to represent alogical one bit state stored in the bitcell 64.

Conversely, if the resistance state ferromagnetic device 70 a has, forexample, an anti-parallel polarization such that the resistance stateferromagnetic device 70 a exhibits a relatively high resistance(compared to that of the complementary resistance state ferromagneticdevice 70 b), and the complementary resistance state ferromagneticdevice 70 b has the complementary parallel polarization such that theresistance state ferromagnetic device 70 b exhibits a relatively lowresistance (compared to that of the resistance state ferromagneticdevice 70 a), the current I_read1 on the bit line BL signal line 230 awill be sensed as relatively low (compared to that of the complementarycurrent I_read2), and the current I_read2 on the complementary bit line/BL signal line 230 b will be sensed as relatively high (compared tothat of the current I_read1), and the bit state of the bitcell 64 willbe sensed, that is read, as representing a logical zero bit state if thehigh resistance state of the resistance state ferromagnetic device 70 ais selected to represent a logical zero bit state stored in the bitcell64.

FIG. 5b depicts one example of a write operation to a supply-switcheddual cell memory bitcell in accordance with the present description. Inone operation, the supply line used for a read operation as discussedabove, is decoupled (block 280, FIG. 5b ) from the common node of thecomplementary resistance state storage cells of the bitcell, and anappropriate write current is driven (block 284) through thecomplementary resistance state storage cells, to write a bit state intothe bitcell.

The polarizations of the pair of complementary resistance stateferromagnetic devices 70 a, 70 b and hence the logical bit value storedin a bitcell 64 of an STTRAM 66 may be set to a particular bit state bypassing a spin polarized current in an appropriate direction through thecomplementary resistance state ferromagnetic devices 70 a, 70 b of thebitcell 64. A spin polarized current is one in which the spinorientations of the charge carriers (such as electrons) arepredominantly of one type, either spin up or spin down.

Thus, a write operation controller logic 290 (FIG. 2) of the controlcircuit 67 is configured to store a logical one in a bitcell 64 of anSTTRAM 66 by passing (block 284, FIG. 5b ) spin polarized currentI_write1 (FIG. 3c ) in one direction (left to right in FIG. 3c ) throughthe complementary resistance state ferromagnetic devices 70 a, 70 b ofthe bitcell 64. As a result, the ferromagnetic layers of the resistancestate ferromagnetic device 70 a of the bitcell 64 have a polarizationwhich is one of parallel or antiparallel, depending upon whichpolarization state has been selected to represent a logical one, and theferromagnetic layers of the complementary resistance state ferromagneticdevice 70 b of the bitcell 64 have a polarization which is the oppositeof that of the resistance state ferromagnetic device 70 a of the bitcell64.

Conversely, a logical zero may be stored in a bitcell 64 of an STTRAM 66by the write operation controller logic 290 (FIG. 2) of the controlcircuit 67 being configured to pass (block 284, FIG. 5b ) spin polarizedcurrent I_write2 (FIG. 3d ) of the same magnitude as the spin polarizedwrite current I_write1, but in the opposite direction (right to left inFIG. 3d ) through the resistance state ferromagnetic devices 70 b, 70 aof the bitcell. As a result, the ferromagnetic layers of the resistancestate ferromagnetic device 70 a of the bitcell 64 have a polarizationwhich is the other of parallel or antiparallel, depending upon whichpolarization has been selected to represent a logical zero, and theferromagnetic layers of the complementary resistance state ferromagneticdevice 70 b of the bitcell 64 have a polarization which is the oppositeof that of the resistance state ferromagnetic device 70 a of the bitcell64. In one embodiment, the write currents I_write1 and I_write2 may havesubstantially the same magnitude since they may be directed through thesame current path but in opposite directions. In other embodiments, themagnitudes may differ, depending upon the particular application.

In one embodiment, in a write operation directed to the bitcell 64, theswitch transistors 69 a, 69 b are active (block 284, FIG. 5b ) and thesupply switch transistor 71 is inactive (block 280, FIG. 5b ).Activation of both switch transistors 69 a and 69 b of resistance statestorage cells 68 a permits the write currents to pass through the switchtransistors 69 a and 69 b and the resistance state ferromagnetic devices70 a, 70 b of the bitcell. Deactivating the supply switch transistor 71decouples (block 280, FIG. 5b ) the supply line 210 from the common node220 and restricts the write current to the dual cells 68 a, 68 b.

In one embodiment, the switch transistors 69 a and 69 b are activated bywrite operation controller logic 290 (FIG. 2) of the memory controlcircuit 67, which is configured to drive the word line WL signal line228 b coupled to the inputs 231 a, 231 b of the switch transistors 69 aand 69 b, respectively, to a true state, which turns the switchtransistors 69 a, 69 b to an ON state, permitting the write current topass through the switch transistors 69 a and 69 b and the resistancestate ferromagnetic devices 70 a, 70 b of the bitcell 64. The supplyswitch transistor 71 is maintained inactive by write operationcontroller logic 290 (FIG. 2) of the memory control circuit 67, which isconfigured to drive the word line read WLR signal line 228 a coupled tothe input 229 of the supply switch transistor 71 to the complementarystate, which turns the supply switch transistor 71 to the OFF state,decoupling the supply line from the common node, thereby preventingwrite current from passing through the supply switch transistor 71during the write operation of the bitcell 64. In addition, the OFF stateof the supply switch transistor 71, also prevents power supply currentfrom the Vc power supply line 210 from passing through the supply switchtransistor 71 during the write operation of the bitcell 64.

The write operation controller logic 290 (FIG. 2) of the control circuit67 is configured to selectively direct a spin polarized write currentthrough the dual cells 68 a, 68 b to write a particular bit state intothe bitcell 64 by providing appropriate signals on the bit line BLsignal line 230 a and the complementary bit line /BL signal line 230 b.Thus, to store a logical one in a bitcell 64 of an STTRAM 66 by passing(block 284, FIG. 5b ) spin polarized current I_write1 (FIG. 3c ) in onedirection (left to right in FIG. 3c ) through the complementaryresistance state ferromagnetic devices 70 a, 70 b of the bitcell 64, thebit line BL signal line 230 a may be raised by the write operationcontroller logic 290 (FIG. 2) to a higher potential than that ofcomplementary bit line /BL signal line 230 b. Conversely, to store alogical zero in a bitcell 64 of an STTRAM 66 by passing (block 284, FIG.5b ) spin polarized current I_write2 (FIG. 3d ) in the other direction(right to left in FIG. 3d ) through the complementary resistance stateferromagnetic devices 70 b, 70 a of the bitcell 64, the complementarybit line /BL signal line 230 b may be raised by the write operationcontroller logic 290 (FIG. 2) to a higher potential than that of the bitline BL signal line 230 a.

It is seen from the above that a supply-switched dual cell memorybitcell 64 of FIGS. 3a-3d may be both read from and written to withoutthe use of source or select SL line signal lines utilized by variousprevious existing and proposed bitcells. For example, FIG. 6 depicts aknown bitcell 600 having a switch transistor 620 and amagnetic-tunneling junction 630 in which a select line SL signal line640 is used in both read and write operations. The bit state of thebitcell 600 is read by providing a read signal on the select line SLsignal line 640, and activating the switch transistor 620 with anactivation signal on a word line WL signal line 650. The bit statesignal on a bit line BL signal line 660 is compared to a referencesignal (not shown). A logical bit value may be written to the bitcell600 by activating the switch transistor 620 with an activation signal onthe word line WL signal line 650, and providing appropriate signals onthe select line SL signal line 640 and the bit line BL signal line 660to selectively direct a spin polarized write current through the MTJ 630to write a particular bit state into the bitcell 600. Thus, to store alogical one in a bitcell 600 of an STTRAM, the bit line BL signal line660 may be raised to a higher potential than that of select line SLsignal line 640. Conversely, to store a logical zero in the bitcell 600of an STTRAM, the select line SL signal line 640 may be raised to ahigher potential than that of bit line BL signal line 660.

FIG. 7 shows another example of a previously proposed bitcell in whichselect line SL signal lines are used in both read and write operations.In this example, a previously proposed bitcell 700 has dual MTJ cells706 a, 706 b, each of which has a switch transistor 720 a, 720 b and amagnetic-tunneling junction 730 a, 730 b in which a select line SLsignal line 740 a and a select line /SL signal line 740 b are used inboth read and write operations. The bit state of the bitcell 700 is readby providing read signals on both select line SL signal line 740 a andcomplementary select line /SL signal line 740 b and by activating theswitch transistors 720 a, 720 b with an activation signal on a word lineWL signal line 750. The bit state signal on a bit line BL signal line760 a is compared to a bit state signal on a complementary bit line /BLsignal line 760 b, using a sense amplifier 770.

A logical bit value may be written to the bitcell 700 by activating theswitch transistors 720 a, 720 b with an activation signal on the wordline WL signal line 750, and providing appropriate signals on selectline SL signal line 740 a, complementary select line /SL signal line 740b, bit line BL signal line 760 a, and bit line /BL signal line 760 b, toselectively direct spin polarized write currents through the MTJs 730 a,730 b to write a particular bit state into the bitcell 700. Thus, tostore a logical one in a bitcell 700 of an STTRAM, the bit line BLsignal line 760 a of the MTJ cell 706 a may be raised to a higherpotential write signal such as V_write, for example, and the select lineSL signal line 740 a of the MTJ cell 706 a lowered to a lower potentialsuch as zero volts, for example. In addition, the select line /SL signalline 740 b of the MTJ cell 706 b may be raised to a higher potentialwrite signal such as V_write, for example, and the bit line /BL signalline 760 b of the MTJ cell 706 b lowered to a lower potential such aszero volts, for example.

Conversely, to store a logical zero in the bitcell 700 of an STTRAM, theselect line SL signal line 740 a of the MTJ cell 706 a may be raised toa higher potential write signal such as V_write, for example, and thebit line BL signal line 760 a of the MTJ cell 706 a lowered to a lowerpotential such as zero volts, for example. In addition, the bit line /BLsignal line 760 b of the MTJ cell 706 b may be raised to a higherpotential write signal such as V_write, for example, and the select line/SL signal line 740 b of the MTJ cell 706 b lowered to a lower potentialsuch as zero volts, for example.

Compared to the single MTJ cell structure of FIG. 6, the dual MTJ cellstructure of the bitcell of FIG. 7 can provide a larger sensing marginfor faster sense and read operation. In addition the complementary bitlines BL, /BL provide complementary bit state information which caneliminate use of a reference voltage or current for read operations.However, both the single MTJ cell structure of the bitcell of FIG. 6,and the dual MTJ cell structure of the bitcell of FIG. 7 utilize one ormore select line SL signal lines for both read and write operations.

FIGS. 8a-8c show another example of a previously proposed bitcell inwhich a select line SL signal line is used in both read and writeoperations. In this example, a previously proposed bitcell 700 a, likethe bitcell 700 of FIG. 7, has dual MTJ cells 706 a, 706 b, each ofwhich has a switch transistor 720 a, 720 b and a magnetic-tunnelingjunction 730 a, 730 b. However, rather than both a select line SL signalline 740 a and a complementary select line /SL signal line 740 b of FIG.7, the bitcell 700 a of FIG. 8a employs a single combined select line SLsignal line 740 in both read and write operations.

The bit state of the bitcell 700 a is read by providing a read signalI_readA on the select line SL signal line 740 coupled at a common node774 of the switch transistors 720 a, 720 b of the MTJ cells 706 a, 706b. In addition, the switch transistors 720 a, 720 b are activated withan activation signal on a read word line RWL signal line 750 a. Theresultant bit state signal I_readA1 on a bit line BL signal line 760 a,is compared to the resultant complementary bit state signal I_readA2 ona complementary bit line /BL signal line 760 b, using a sense amplifiersuch as the amplifier 770 of FIG. 7.

Referring to FIGS. 8b, 8c , write operations directed to the bitcell 700a include activating a third switch transistor 780 which is coupled atone end to a node 782 between the MTJ 730 a and the switch transistor720 a, and is coupled at its other end to a node 784 between the switchtransistor 720 b and the MTJ 730 b. When activated, the switchtransistor 780 bypasses the switch transistors 720 a, 720 b. The switchtransistor 780 is activated with an activation signal on a write wordline WWL signal line 750 b. To store a bit state in the bitcell 700 a,the switch transistor 780 is activated and appropriate signals areprovided on bit line BL signal line 760 a, and complementary bit line/BL signal line 760 b, to selectively direct spin polarized writecurrents through the MTJs 730 a, 730 b to write a particular bit stateinto the bitcell 700.

Thus, to store a logical one in a bitcell 700 a of an STTRAM, the bitline BL signal line 760 a of the MTJ cell 706 a may be raised to ahigher potential write signal such as V_write, for example, and thecomplementary bit line /BL signal line 760 b of the bitcell 700 alowered to a lower potential such as zero volts, for example. As aresult, a spin polarized write current I_writeA1 (FIG. 8b ) passesthrough (in a left to right direction in the view of FIG. 8b ) throughthe MTJ 730 a, through the switch transistor 780 and through the MJT 730b to write a particular bit state such as a logical one value, into thebitcell 700 a.

Conversely, to store a logical zero in the bitcell 700 a of an STTRAM,the complementary bit line /BL signal line 760 b of the MTJ cell 706 bmay be raised to a higher potential write signal such as V_write, forexample, and the bit line BL signal line 760 a of the MTJ cell 706 a maybe lowered to a lower potential such as zero volts, for example. As aresult, a spin polarized write current I_writeA2 (FIG. 8c ) passesthrough (in a right to left direction in the view of FIG. 8c ) throughthe MTJ 730 b, through the switch transistor 780 and through the MJT 730a to write a particular bit state such as a logical zero value, into thebitcell 700 a.

Compared to the dual MTJ cell structure of the bitcell of FIG. 7, thedual MTJ cell structure of the bitcell of FIGS. 8a-8c may have a lowerwrite current since both MTJs 730 a, 730 b are in the write pathI_writeA1 (FIG. 8b ) or write path I_writeA2 (FIG. 8c ). In addition,the number of column oriented signal lines is reduced from four signallines (SL, /SL, BL, /BL) in the bitcell of FIG. 7, to three columnoriented signal lines, (SL, BL, /BL) in the bitcell of FIGS. 8a-8c .However, in a bitcell in accordance with one aspect of the presentdescription such as bitcell 64 (FIGS. 3a-3d ), both column orientedsignal lines SL, /SL can be eliminated, reducing the number of columnoriented signal lines to two (BL, /BL) as shown for the bitcell 64 ofFIGS. 3a-3d , for example. As a result, layout restrictions of circuitelements may be relaxed, which may facilitate cell size shrinkage ascircuit fabrication processes advance.

In addition, in a bitcell in accordance with another aspect of thepresent description, the supply line such as the Vc power supply line210 (FIGS. 3a-3d ) may be a mesh of supply lines in the form of a gridto provide increased reliability and stability of bitcell operationssuch as read operations, as compared to many previous bitcell designs.For example, it is appreciated herein that in the bitcell 700 a of FIG.8a , the read current I_readA flowing through the combined select lineSL signal line 740 may be almost double the read current of otherbitcells. As a result, the width of the combined select line SL signalline 740 may be substantially wider than prior select line SL signallines to reduce resistance of the select line SL signal line 740 toaccommodate the increased read current.

It is further appreciated herein that use of select line SL signal linesin a memory can have an adverse effect upon read margins. FIG. 9 showsan example of a memory array 910 of bitcells 700 a similar to the priorbitcells 700 a of FIGS. 8a-8c , utilizing columnar select line SL signallines such as select line SL signal line 740 a 1, for example, adjacenta column of bitcells 700 a including a bitcell 700 a 1, for example, andselect line SL signal line 740 a 2, for example, adjacent a column ofbitcells 700 a including a bitcell 700 a 2, for example. (The selectline SL signal lines 740 (FIG. 8a ) for the remaining columns of thearray 910 of bitcells 700 a are omitted for purposes of clarity.) Alsodepicted are row read word line RWL signal lines such as row read wordline RWL signal line 750 a 1, for example, adjacent a row of bitcells700 a including the bitcell 700 a 1, for example, and row read word lineRWL signal line 750 a 2, for example, adjacent a row of bitcells 700 aincluding a bitcell 700 a 2, for example. (The complementary column bitline /BL (FIG. 8a ) signal lines and row write word line WWL (FIG. 8a )signal lines are omitted from FIG. 9 for purposes of clarity.)

As shown in FIG. 9, the positions of the bitcells 700 a 1 and 700 a 2are different within the array 910. Thus, the length along the selectline SL signal line 740 a 1 from a select line SL driver 912 a to thebitcell 700 a 1 is significantly longer than the length along the selectline SL signal line 740 a 2 from a select line SL driver 912 b to thebitcell 700 a 2. Hence, the parasitic resistance of that portion of theselect line SL signal line 740 a 1 from the select line SL driver 912 ato the bitcell 700 a 1 as represented by the resistor R1 depicted inphantom, is substantially larger than the parasitic resistance of thatportion of the select line SL signal line 740 a 2 from the select lineSL driver 912 b to the bitcell 700 a 2 as represented by the resistor R2depicted in phantom. In a similar manner, the parasitic resistance ofthat portion of the bit line BL signal line 760 a 1 from the bitcell 700a 1 to a sense amplifier 914 a as represented by the resistor R3depicted in phantom, is substantially larger than the parasiticresistance of that portion of the bit line BL signal line 760 a 2 fromthe bitcell 700 a 2 to a sense amplifier 914 b as represented by theresistor R4 depicted in phantom.

It is seen that the total resistance of the read signal path through theparasitic resistance R1, the bitcell 700 a 1 and the parasiticresistance R3 is substantially higher than that of the read signal paththrough the parasitic resistance R2, the bitcell 700 a 2 and theparasitic resistance R4, notwithstanding that the resistances of the MTJcells 706 a, 706 b (FIG. 8a ) of the bitcells 700 a 1, 700 a 2 may besubstantially identical. These differences in total resistances in theread paths as a function of bitcell position within the array 910 maycause degradation in read margins and may cause read accuracy failures.Such degradation and failures may be further aggravated by variations inresistance due to fabrication process variations.

As explained in greater detail below, by eliminating the SL signal linein an array of supply-switched dual cell memory bitcells 64 inaccordance with the present description, the layout restrictions ofcircuit elements may be relaxed, which may facilitate cell sizeshrinkage as circuit fabrication processes advance. In addition, thesupply line may be a mesh of supply lines in the form of a grid ofsupply lines to provide increased reliability and stability of bitcelloperations such as read operations.

FIG. 10 shows an example of a memory array 1010 of bitcells 64 similarto the bitcells 64 of FIGS. 3s-3d , in which columnar select line SLsignal lines of prior bitcells such as the select line SL signal linesof FIG. 9, have been eliminated. In one aspect of a memory employingsupply-switched dual cell memory bitcells 64 in accordance with thepresent description, the Vc power supply line 210 (FIG. 3b ) is depictedin FIG. 10 in the form of an orthogonal mesh or grid 210 of a set ofcolumnar power supply lines 210 a interconnected at nodes 1020 with aset of row power supply lines 210 b as shown in FIG. 10. The orthogonalsets of power supply lines 210 a, 210 b of the power supply line grid210 are substantially equipotential at a power supply voltage Vcprovided by one or more power supplies 1030 a 1, 1030 a 2 which may be aconstant current or constant voltage power supply, for example.

In one embodiment, a columnar power supply line 210 a may be providedadjacent each column of bitcells 64 of the array 1010. For example, acolumnar power supply line 210 a 1 of the set of parallel, spaced powersupply lines 210 a may be provided adjacent a column of bitcells 64including a bitcell 64 a 1, for example, and a columnar power supplyline 210 a 2 of the set of parallel, spaced power supply lines 210 a maybe provided adjacent a column of bitcells 64 including a bitcell 64 a 2,for example. In addition, a row power supply line of a set of parallel,spaced row power supply lines 210 b may be provided adjacent each row ofbitcells 64 of the array 1010. For example, a row power supply line 210b 1 of the set of row power supply lines 210 b may be provided adjacenta row of bitcells 64 including the bitcell 64 a 1, for example, and arow power supply line 210 b 2 of the set of row power supply lines 210 bmay be provided adjacent a row of bitcells 64 including the bitcell 64 a2, for example.

In this embodiment, the set of columnar power supply lines 210 a isarranged in a column direction and the set of row power supply lines 210b is arranged in a row direction orthogonal to the column direction ofthe set of power supply lines 210 a. In one aspect of the presentdescription, it is appreciated that the grid-like structure of the powersupply lines 210 a, 210 b not only reduces the overall resistance of thegrid 210 of power supply lines, but also increases the uniformity of theresistances of the various current paths through the grid 210,notwithstanding differences in array position of the bitcells 64. Inaddition, it is appreciated that the grid-like structure of the powersupply lines 210 a, 210 b increases the overall capacitance of the powersupply lines 210 a, 210 b which can increase the stability of the powersupply 1030 a 1, 1030 a 2. As a result, it is appreciated that overallaccuracy and stability of read operations may be enhanced as well.

Also depicted are examples of a set of parallel, spaced bit line BL(FIG. 3a ) signal lines including a column bit line BL signal line 238 a1, for example, adjacent a column of bitcells 64 including the bitcell64 a 1, for example, and a column bit line BL signal line 238 a 2, forexample, adjacent the column of bitcells 64 including the bitcell 64 a2, for example. (The column bit line BL signal lines of the set of bitline BL signal lines for the remaining columns of the array 1010 ofbitcells 64 are omitted for purposes of clarity.)

Also depicted are examples of a set of parallel, spaced row read wordline WLR (FIG. 3a ) signal lines including row read word lines such asrow read word line WLR signal line 228 a 1, for example, adjacent a rowof bitcells 64 including the bitcell 64 a 1, for example, and row readword line WLR signal line 228 a 2, for example, adjacent a row ofbitcells 64 including the bitcell 64 a 2, for example. (The row readword line WLR signal lines for the remaining rows of the array 1010 ofbitcells 64 are omitted for purposes of clarity.)

In this embodiment, the sets of columnar bit line BL and /BL signallines are arranged in a column direction and the sets of row read andwrite word line WLR, WLW signal lines are arranged in a row directionorthogonal to the column direction of the sets of bit line signal lines.(The set of column complementary bit line /BL (FIG. 3a ) signal linesand the set of row write word line WWL (FIG. 3a ) signal lines areomitted from FIG. 10 for purposes of clarity.)

As shown in FIG. 10, the positions of the bitcells 64 a 1 and 64 a 2 aredifferent within the array 910. However, due to the grid structure ofthe grid 210 of power supply lines 210 a, 210 b, the lengths of thecurrent paths from a power supply such as power supply 1030 a 1, throughthe grid 210 to the bitcell 64 a 1 may have increased uniformity withrespect to the lengths of the current paths from a power supply such aspower supply 1030 a 2, through the grid 210 to the bitcell 700 a 2, forexample. Hence, the parasitic resistances of the grid 210 may not onlybe reduced as noted above as compared to those of the SL signal lines ofprior bitcells, but may also be more uniform as compared to those of theSL signal lines of prior bitcells, notwithstanding differences inbitcell position within the array 1010. Hence, due to increaseduniformity of parasitic resistances of current paths through the grid210, the overall parasitic resistances of a read current path throughthe grid 210 to the bitcell 64 a 1, for example, and through theparasitic resistance R5 of the bit line BL signal line 238 a 1 to asense amplifier 1040 a 1, may have increased uniformity with respect tothe overall parasitic resistances of a read current path through thepower supply line grid 210 to the bitcell 64 a 2 and through theparasitic resistance R6 of the bit line BL signal line 238 a 2 to asense amplifier 1040 a 2, notwithstanding the difference in parasiticresistances between the parasitic resistance R5 of the bit line BLsignal line 238 a 1, and the parasitic resistance R6 of the bit line BLsignal line 238 a 2. The increased uniformity in total resistance in theread paths notwithstanding bitcell position within the array 1010 mayenhance read margins and accuracy in read operations. Such readoperation enhancements may increase toleration to variations inresistance due to fabrication process variations.

FIG. 11a-11d are directed to another embodiment of a supply-switcheddual cell memory bitcell 1164 in accordance with the presentdescription. In this embodiment, each bitcell 1164 of the array 60 (FIG.2) of bitcells 1164, like the bitcells 64 of FIGS. 3a-3d , includes acomplementary pair of resistance state storage cells 1168 a, 1168 b(FIG. 11a-11d ), each of which includes a storage cell switch transistor1169 a, 1169 b series connected to a resistance state ferromagneticdevice 1170 a, 1170 b such as a spin valve, or a magnetic tunneljunction (MTJ) device as shown in FIG. 11a . However, in thisembodiment, the positions of the series-connected switch transistors1169 a, 1169 b and resistance state ferromagnetic device 1170 a, 1170 bare switched relative to the positions of the series-connected switchtransistors 69 a, 69 b and resistance state ferromagnetic device 70 a,70 b of the complementary pair of resistance state storage cells 68 a,68 b (FIG. 3a-3d ). Thus, in this embodiment, the switch transistors 69a, 69 b are connected to a common node 220 a, and the resistance stateferromagnetic devices (MTJs) 70 a, 70 b are connected to bit line signallines BL 230 a, /BL 230 b, respectively, as shown in FIGS. 11a -11 d.

Like the bitcell 64 of FIGS. 3a-3d , the bitcell 1164 of FIGS. 11a-11dhas a supply switch transistor 71 which selectively couples a supplyline such as a Vc constant voltage or current line 210 to the dual cells1168 a, 1168 b for read operations as explained in greater detail below.Also, like the bitcell 64 of FIGS. 3a-3d , the supply-switched dual cellmemory bitcell 1164 of FIGS. 11a-11d has a reduced number of signallines for storing and reading a data bit. In the embodiment of FIGS.11a-11d , the bitcell 1164 lacks a source-line or select line (SL)signal line typical of many previous existing and proposed bitcells.

The read and write operations of the supply-switched dual cell memorybitcell 1164 of FIGS. 11a-11d are similar to those described above inconnection with the supply-switched dual cell memory bitcell 64 of FIGS.3a-3d . Accordingly, in one operation, a supply line such as Vc powersupply line 210 (FIG. 3b ) is coupled (block 214, FIG. 5a ) to a commonnode 220 (FIG. 3b ) of the bitcell 64 by activation of the supply switchtransistor 71. Here too, in one embodiment, the supply switch transistor71 is activated by read operation controller logic 212 (FIG. 2) of thememory control circuit 67, configured to drive a word line read (WLR)signal line 228 a coupled to an input 229 of the supply switchtransistor 71, to a true state, which turns the supply switch transistor71 to an ON state, permitting a constant power current I_power (FIG. 11b) to flow from the Vc power supply line 210 through the supply switchtransistor 71 to the common node 220 a.

In response, first and second complementary read currents, I_read1,I_read2, (I_read1+I_read2=I_power), are driven (block 232, FIG. 5a )from the common node 220 a, through first and second complementaryresistance state storage cells, 1168 a, 1168 b, respectively, to a pairof first and second complementary bit line BL, /BL signal lines 230 a,230 b, respectively, of the bitcell 1164.

In one embodiment, the storage cell switch transistor 1169 a isactivated by read operation controller logic 212 (FIG. 2) of the memorycontrol circuit 67, which is configured to drive a word line WL signalline 228 b coupled to an input 231 a of the storage cell switchtransistor 1169 a, to a true state, which turns the storage cell switchtransistor 1169 a to an ON state, permitting read current I_read1 toflow through the storage cell switch transistor 1169 a and theresistance state ferromagnetic device (MTJ) 1170 a to the bit line BLsignal line 230 a. The read current I_read1 on the bit line BL signalline 230 a provides a bit state signal on the bit line BL signal line230 a. Thus, in the embodiment of FIGS. 11a, 11b , the storage cellswitch transistor 1169 a is coupled at one end to the common node 220 aand coupled at the other end to one end of the MTJ 1170 a, which iscoupled at its other end to the bit line BL signal line 230 a of thebitcell 1164.

Similarly, activation of storage cell switch transistor 1169 b of thesecond resistance state storage cell permits the complementary readcurrent I_read2 to pass through the series-connected storage cell switchtransistor 1169 b and the MTJ 1170 b of the complementary resistancestate storage cell 1168 b, to the complementary bit line /BL signal line230 b of the bitcell 1164. Thus, the resistance state storage cell 1168b has an input coupled to the common node 220 a and an output coupled tothe bit line /BL signal line 230 b of the bitcell 1164. Morespecifically, the series-connected storage cell switch transistor 1169 band MTJ 1170 b of the resistance state storage cell 1168 b are connectedbetween the common node 220 a and the bit line /BL signal line 230 b ofthe bitcell 1164.

In one embodiment, the storage cell switch transistor 1169 b isactivated by read operation controller logic 212 (FIG. 2) of the memorycontrol circuit 67, which is configured to drive the word line WL signalline 228 b coupled to an input 231 b of the storage cell switchtransistor 1169 b, to a true state, which turns the storage cell switchtransistor 1169 b to an ON state, permitting read current I_read2 toflow through the storage cell switch transistor 1169 b and theresistance state ferromagnetic device (MTJ) 1170 b to the bit line /BLsignal line 230 b. The read current I_read2 on the complementary bitline /BL signal line 230 b provides a complementary state signal on thecomplementary bit line /BL signal line 230 b. Thus, in the embodiment ofFIGS. 11a-11d , the storage cell switch transistor 1169 b is coupled atone end to the common node 220 a and coupled at the other end to one endof the MTJ 1170 b, which is coupled at its other end to thecomplementary bit line /BL signal line 230 b of the bitcell 1164. Thecomplementary bit state signals represented by the complementary readcurrents I_read1, I_read2, on the complementary bit line BL, /BL signallines, 230 a, 230 b, respectively are sensed (block 240, FIG. 5a ) by asuitable sense amplifier.

FIG. 5b also depicts one example of a write operation to asupply-switched dual cell memory bitcell 1164 in accordance with thepresent description. In one operation, the supply line used for a readoperation as discussed above, is decoupled (block 280, FIG. 5b ) fromthe common node of the complementary resistance state storage cells ofthe bitcell, and an appropriate write current is driven (block 284)through the complementary resistance state storage cells, to write a bitstate into the bitcell.

Thus, a write operation controller logic 290 (FIG. 2) of the controlcircuit 67 is configured to store a logical one in a bitcell 1164 of anSTTRAM such as the STTRAM 66 by passing (block 284, FIG. 5b ) spinpolarized current I_write1 (FIG. 11c ) in one direction (right to leftin FIG. 11c ) through the complementary resistance state ferromagneticdevices 1170 a, 1170 b of the bitcell 64. As a result, the ferromagneticlayers of the resistance state ferromagnetic device 1170 a of thebitcell 1164 have a polarization which is one of parallel orantiparallel, depending upon which polarization state has been selectedto represent a logical one, and the ferromagnetic layers of thecomplementary resistance state ferromagnetic device 1170 b of thebitcell 1164 have a polarization which is the opposite of that of theresistance state ferromagnetic device 1170 a of the bitcell 1164.

Conversely, a logical zero may be stored in a bitcell 1164 of an STTRAM66 by the write operation controller logic 290 (FIG. 2) of the controlcircuit 67 being configured to pass (block 284, FIG. 5b ) spin polarizedcurrent I_write2 (FIG. 11d ) in the opposite direction (right to left inFIG. 11d ) through the resistance state ferromagnetic devices 1170 a,1170 b of the bitcell. As a result, the ferromagnetic layers of theresistance state ferromagnetic device 1170 a of the bitcell 1164 have apolarization which is the other of parallel or antiparallel, dependingupon which polarization has been selected to represent a logical zero,and the ferromagnetic layers of the complementary resistance stateferromagnetic device 1170 b of the bitcell 1164 have a polarizationwhich is the opposite of that of the resistance state ferromagneticdevice 1170 a of the bitcell 1164.

In one embodiment, in a write operation directed to the bitcell 1164,the switch transistors 1169 a, 1169 b are active (block 284, FIG. 5b )and the supply switch transistor 71 is inactive (block 280, FIG. 5b ).Activation of both switch transistors 1169 a and 1169 b of resistancestate storage cells 1168 a permits the write currents to pass throughthe switch transistors 1169 a and 1169 b and the resistance stateferromagnetic devices 1170 a, 1170 b of the bitcell. Deactivating thesupply switch transistor 71 decouples (block 280, FIG. 5b ) the supplyline 210 from the common node 220 a and restricts the write current tothe dual cells 1168 a, 1168 b.

In one embodiment, the switch transistors 1169 a and 1169 b areactivated by write operation controller logic 290 (FIG. 2) of the memorycontrol circuit 67, which is configured to drive the word line WL signalline 228 b coupled to the inputs 231 a, 231 b of the switch transistors1169 a and 1169 b, respectively, to a true state, which turns the switchtransistors 1169 a, 1169 b to an ON state, permitting the write currentpass through the switch transistors 1169 a and 1169 b and the resistancestate ferromagnetic devices 1170 a, 1170 b of the bitcell 1164. Thesupply switch transistor 71 is maintained inactive by write operationcontroller logic 290 (FIG. 2) of the memory control circuit 67, which isconfigured to drive the word line read WLR signal line 228 a coupled tothe input 229 of the supply switch transistor 71 to the complementarystate, which turns the supply switch transistor 71 to the OFF state,preventing write current from passing through the supply switchtransistor 71 during the write operation of the bitcell 1164. Inaddition, the OFF state of the supply switch transistor 71, alsoprevents power supply current from the Vc power supply line 210 frompassing through the supply switch transistor 71 during the writeoperation of the bitcell 1164.

The write operation controller logic 290 (FIG. 2) of the control circuit67 is configured to selectively direct a spin polarized write currentthrough the dual cells 1168 a, 1168 b to write a particular bit stateinto the bitcell 1164 by providing appropriate signals on the bit lineBL signal line 230 a and the complementary bit line /BL signal line 230b. Thus, to store a logical one in a bitcell 1164 of an STTRAM 66 bypassing (block 284, FIG. 5b ) spin polarized current I_write1 (FIG. 11c) in one direction (left to right in FIG. 11c ) through thecomplementary resistance state ferromagnetic devices 1170 a, 1170 b ofthe bitcell 1164, the bit line BL signal line 230 a may be raised by thewrite operation controller logic 290 (FIG. 2) to a higher potential thanthat of complementary bit line /BL signal line 230 b. Conversely, tostore a logical zero in a bitcell 1164 of an STTRAM 66 by passing (block284, FIG. 5b ) spin polarized current I_write2 (FIG. 11d ) in the otherdirection (right to left in FIG. 11d ) through the complementaryresistance state ferromagnetic devices 1170 b, 1170 a of the bitcell1164, the complementary bit line /BL signal line 230 b may be raised bythe write operation controller logic 290 (FIG. 2) to a higher potentialthan that of the bit line BL signal line 230 a.

It is seen from the above that a supply-switched dual cell memorybitcell 1164 of FIGS. 11a-11d may be both read from and written towithout the use of source or select SL line signal lines utilized byvarious previous existing and proposed bitcells. In another aspect of amemory employing supply-switched dual cell memory bitcells 1164 inaccordance with the present description, the supply line 210 (FIGS.11a-11d ) may be provided in the form of an orthogonal mesh or grid in amanner similar to that depicted in FIG. 10 for the grid 210 (FIG. 10) ofa set of columnar power supply lines 210 a interconnected at nodes 1020with a set of row power supply lines 210 b in connection with thesupply-switched dual cell memory bitcells 64. In the embodiments ofFIGS. 3a-3d , 10 and 11 a-11 d, a Vc power supply line such as aconstant voltage or constant current line 210 is coupled by the supplyswitch 71 to the dual cells of the bitcell for read operations. However,it is appreciated that the supply line 210 may be a ground (GND) supplyline, depending upon the particular application.

For example, FIG. 12 shows an embodiment in which in one read operation,a supply line such as GND supply line 210 c (FIG. 12) is coupled (block214, FIG. 5a ) to a common node 220 c (FIG. 12) of the bitcell 1264 byactivation of the supply switch transistor 71 which is configured tocouple the common node 220 c to the supply when activated. In thisembodiment, the supply switch transistor 71 is activated by readoperation controller logic 212 (FIG. 2) of the memory control circuit 67which is configured to activate the supply switch transistor 71 bydriving a word line read (WLR) signal line 228 a (FIG. 12) coupled to aninput 229 of the supply switch transistor 71, to a true state, whichturns the supply switch transistor 71 to an ON state, permitting aconstant power current I_powerA to flow from the common node 220 c,through the supply switch transistor 71, to the ground (GND) supply line210 c.

Also, first and second complementary read currents, I_read1 a, I_read2a, (I_read1 a+I_read2 a=I_powerA), are driven (block 232, FIG. 5a )towards the common node 220 c, through first and second complementaryresistance state storage cells, 68 a, 68 b, respectively, from a pair offirst and second complementary bit line BL, /BL signal lines 230 a, 230b, respectively, of the bitcell 1264. In one embodiment, activation ofstorage cell switch transistor 69 a of the first resistance statestorage cell 68 a permits the read current I_read1 a to pass from thebit line BL signal line 230 a, through the series-connected storage cellswitch transistor 69 a and MTJ 70 a of the resistance state storage cell68 a, of the bitcell 1264.

In this embodiment, the storage cell switch transistor 69 a is activatedby read operation controller logic 212 (FIG. 2) of the memory controlcircuit 67, which is configured to drive a word line WL signal line 228b coupled to an input 231 a of the storage cell switch transistor 69 a,to a true state, which turns the storage cell switch transistor 69 a toan ON state, permitting read current I_read1 a to flow from the bit lineBL signal line 230 a, through the storage cell switch transistor 69 aand the resistance state ferromagnetic device (MTJ) 70 a to the commonnode 220 c. The read current I_read1 a on the bit line BL signal line230 a provides a bit state signal on the bit line BL signal line 230 a.

Similarly, activation of storage cell switch transistor 69 b of thefirst resistance state storage cell permits the complementary readcurrent I_read2 a to pass from the complementary bit line /BL signalline 230 b, through the series-connected storage cell switch transistor69 b and MTJ 70 b of the complementary resistance state storage cell 68b, to the common node 220 c of the bitcell 1264.

In this embodiment, the storage cell switch transistor 69 b is activatedby read operation controller logic 212 (FIG. 2) of the memory controlcircuit 67, which is configured to drive the word line WL signal line228 b coupled to an input 231 b of the storage cell switch transistor 69b, to a true state, which turns the storage cell switch transistor 69 bto an ON state, permitting read current I_read2 a to flow from the bitline /BL signal line 230 b, through the storage cell switch transistor69 b and resistance state ferromagnetic device (MTJ) 70 b, and to thecommon node 220 c. The read current I_read2 a on the complementary bitline /BL signal line 230 b provides a complementary state signal on thecomplementary bit line /BL signal line 230 b.

The complementary bit state signals represented by the complementaryread currents I_read1 a, I_read2 a, on the complementary bit line BL,/BL signal lines, 230 a, 230 b, respectively are sensed, that is read(block 240, FIG. 5a ) by a suitable sense amplifier. In another aspectof a memory employing supply-switched dual cell memory bitcells 1264 inaccordance with the present description, the ground supply line 210 c(FIG. 12) may be provided in the form of an orthogonal mesh or grid in amanner similar to that depicted in FIG. 10 for the grid 210 (FIG. 10).Thus, a grid 210 c of ground supply lines may include a set of columnarground supply lines interconnected at nodes with a set of row groundsupply lines in a manner described above in connection with the powersupply-switched dual cell memory bitcells 64.

EXAMPLES

The following examples pertain to further embodiments.

Example 1 is apparatus, comprising: a plurality of signal linesincluding a pair of complementary bit line signal lines, a first wordline signal line and a second word line signal line, a supply line, anda bitcell configured to store a bit state, said bitcell including acommon node and first and second complementary resistance state storagecells, each resistance state storage cell having an input coupled to thecommon node and an output coupled to a complementary bit line signalline, each complementary resistance state storage cell having a storagecell switch transistor having an input coupled to the first word linesignal line, and a resistance state ferromagnetic device coupled inseries with the storage cell switch transistor of the complementaryresistance state storage cell and having a resistance statecomplementary to a resistance state of the resistance stateferromagnetic device of the complementary resistance state storage cell,and a supply switch transistor having an input coupled to the secondword line signal line and configured to couple the common node to thesupply line when activated.

In Example 2, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include a Spin Transfer Torque Random AccessMemory (STTRAM) which includes a plurality of said bitcells arranged ina rectangular array, wherein a resistance state ferromagnetic device ofeach resistance state storage cell includes a magnetic-tunnelingjunction (MTJ).

In Example 3, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include a memory controller configured tocontrol read and write operations directed to the bitcell, said memorycontroller having read operation controller logic configured to read thebit state of the bitcell, said read operation controller logic beingconfigured to activate the storage cell switch transistors of thecomplementary resistance state storage cells and the supply switchtransistor of the bitcell to cause the bitcell to provide complementarybit state signals on the complementary bit line signal lines wherein thebit state of the bitcell is read.

In Example 4, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include wherein the memory controller furtherhas write operation controller logic configured to write a bit stateinto the bitcell, said write operation controller logic being configuredto activate the storage cell switch transistors of the complementaryresistance state storage cells and maintain the supply switch transistorof the bitcell as inactive to decouple the common node from the supplyline, and to selectively direct one of a first write current in a firstdirection through the resistance state storage cells to write a firstbit state into the bitcell, and a second write current in a seconddirection opposite the first direction through the resistance statestorage cells to write a second bit state opposite the first bit stateinto the bitcell.

In Example 5, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include wherein a first resistance stateferromagnetic device of the first resistance state storage cell is afirst magnetic-tunneling junction (MTJ) and a second resistance stateferromagnetic device of the second resistance state storage cell is asecond magnetic-tunneling junction (MTJ).

In Example 6, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include wherein a first storage cell switchtransistor and a first MTJ of the first resistance state storage cellare series connected between the common node and a bit line signal line,a second storage cell switch transistor and a first second of the secondresistance state storage cell are series connected between the commonnode and a bit line signal line.

In Example 7, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include a supply which is one of a power supplyand a ground supply coupled to the supply line, a memory which includesa plurality of said bitcells arranged in a rectangular array, and anorthogonal grid of supply lines including a first set of parallel spacedsupply lines coupled to the supply and arranged in a first directionwith respect to the rectangular array of bitcells, and a second set ofparallel spaced supply lines coupled to the supply and arranged in asecond direction orthogonal to the first direction of the first set ofparallel spaced supply lines.

In Example 8, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include wherein the pair of complementary bitline signal lines are arranged in a first set of parallel spaced bitline signal lines in a first direction, and the first and second wordline signal lines are arranged in a second set of parallel spaced wordline signal lines in a second direction orthogonal to the firstdirection of the first set of bit line signal lines.

In Example 9, the subject matter of Examples 1-9 (excluding the presentExample) can optionally include a computing system comprising: a memory,a processor configured to cause a data write into or data read from thememory, and any of: a display communicatively coupled to the processor,a network interface communicatively coupled to the processor, or abattery coupled to provide power to the system, wherein the memoryincludes a magnetoresistive random access memory (MRAM), comprising:said pair of complementary bit line signal lines, said first word linesignal line and said second word line signal line, said supply line, andan array of said bitcells, each of which is an MRAM bitcell.

Example 10 is a method, comprising: reading a bit state in a bitcellincluding: coupling a supply line to a common node of the bitcell todrive first and second complementary currents through first and secondcomplementary resistance state storage cells, respectively, of thebitcell, to a pair of first and second complementary bit line signallines, respectively, of the bitcell, each resistance state storage cellhaving one of a first and second resistance state ferromagnetic devicecoupled between the common node and a bit line signal line of the firstand second bit line signal lines, each resistance state ferromagneticdevice having a resistance state complementary to a resistance state ofthe resistance state ferromagnetic device of the complementaryresistance state storage cell, wherein the first and secondcomplementary currents are driven through the first and secondresistance state ferromagnetic devices of the first and secondcomplementary resistance state storage cells, respectively, and sensingcomplementary bit state signals on the pair of first and secondcomplementary bit line signal lines.

In Example 11 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include wherein each of the first andsecond resistance state ferromagnetic devices of the first and secondcomplementary resistance state storage cells, respectively, includes amagnetic-tunneling junction (MTJ) and wherein a plurality of saidbitcells are arranged in a rectangular array of a Spin Transfer TorqueRandom Access Memory (STTRAM).

In Example 12 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include using read operation controllerlogic of a memory controller to read the bit state of the bitcell,wherein the bit state reading further includes activating first andsecond storage cell switch transistors of the first and secondcomplementary resistance state storage cells, respectively, to permitthe first and second complementary currents to pass through the firstand second complementary resistance state storage cells, respectively,and activating a supply switch transistor of the bitcell to couple thesupply line to the common node of the bitcell to drive the first andsecond complementary currents through the first and second complementaryresistance state storage cells, respectively, to cause the bitcell toprovide complementary bit state signals on the complementary bit linesignal lines wherein the bit state of the bitcell is read.

In Example 13 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include using write operation controllerlogic of the memory controller to write a bit state into the bitcell,wherein the bit state writing includes: activating the first and secondstorage cell switch transistors of the complementary resistance statestorage cells, maintaining the supply switch transistor of the bitcellas inactive to decouple the supply line from the common node of thecomplementary resistance state storage cells of the bitcell, andselectively directing one of a first write current in a first directionthrough the resistance state storage cells to write a first bit stateinto the bitcell, and a second write current in a second directionopposite the first direction through the resistance state storage cellsto write a second bit state opposite the first bit state into thebitcell.

In Example 14 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include wherein the first resistancestate ferromagnetic device of the first resistance state storage cell isa first magnetic-tunneling junction (MTJ) and the second resistancestate ferromagnetic device of the second resistance state storage cellis a second magnetic-tunneling junction (MTJ).

In Example 15 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include wherein driving first and secondcomplementary currents through first and second complementary resistancestate storage cells, respectively includes driving the firstcomplementary current through the first MTJ and the first storage cellswitch transistor which are series connected between the common node andthe first bit line signal line, and driving the second complementarycurrent through the second MTJ and the second storage cell switchtransistor which are series connected between the common node and thesecond bit line signal line.

In Example 16 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include wherein the bitcell is a bitcellof a plurality of bitcells arranged in a rectangular array, and whereinthe coupling a supply line to a common node of the bitcell furthercomprises coupling a supply line of an orthogonal grid of supply linesincluding a first set of parallel spaced supply lines coupled to asupply and arranged in a first direction with respect to the rectangulararray of bitcells, and a second set of parallel spaced supply linescoupled to the supply and arranged in a second direction orthogonal tothe first direction of the first set of parallel spaced supply lineswherein the supply lines are one of power supply lines and ground supplylines.

In Example 17 the subject matter of Examples 10-17 (excluding thepresent Example) can optionally include wherein the sensingcomplementary bit state signals on the pair of first and secondcomplementary bit line signal lines includes sensing the complementarybit state signals on the pair of first and second complementary bit linesignal lines arranged in a first set of parallel spaced bit line signallines in a first direction, and wherein the activating first and secondstorage cell switch transistors of the first and second complementaryresistance state storage cells includes driving signals on first andsecond word line signal lines arranged in a second set of parallelspaced word line signal lines in a second direction orthogonal to thefirst direction of the first set of bit line signal lines.

Example 18 is a memory apparatus comprising means to perform a method asclaimed in any preceding Example.

Example 19 is a computing system, comprising: a memory, and a processorconfigured to cause a data write into or data read from the memory,wherein the memory includes a magnetoresistive random access memory(MRAM), comprising: a pair of complementary bit line signal lines, afirst word line signal line and a second word line signal line, an arrayof MRAM bitcells, each bitcell configured to store a bit state, eachbitcell including: a common node, first and second complementaryresistance state storage cells, each resistance state storage cellhaving an input coupled to the common node, an output coupled to acomplementary bit line signal line, a storage cell switch transistorhaving an input coupled to the first word line signal line, and aresistance state ferromagnetic device coupled in series with the storagecell switch transistor of the resistance state storage cell and having aresistance state complementary to a resistance state of the resistancestate ferromagnetic device of the complementary resistance state storagecell, wherein each resistance state ferromagnetic device has apolarization which in a first state is one of parallel and anti-parallelpolarization and in a second state is the other of parallel andanti-parallel polarization, a supply line which is one of a power supplyline and a ground supply line, and a supply switch transistor having aninput coupled to the second word line signal line and configured tocouple the common node to the supply line when activated.

In Example 20 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein the magnetoresistiverandom access memory is a Spin Transfer Torque Random Access Memory(STTRAM) which includes a plurality of said bitcells arranged in arectangular array, wherein a resistance state ferromagnetic device ofeach resistance state storage cell includes a magnetic-tunnelingjunction (MTJ).

In Example 21 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include a memory controller configuredto control read and write operations directed to the bitcell, saidmemory controller having read operation controller logic configured toread the bit state of the bitcell, said read operation controller logicbeing configured to activate the storage cell switch transistors of thecomplementary resistance state storage cells and the supply switchtransistor of the bitcell to cause the bitcell to provide complementarybit state signals on the complementary bit line signal lines wherein thebit state of the bitcell is read.

In Example 22 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein the memory controllerfurther has write operation controller logic configured to write a bitstate into the bitcell, said write operation controller logic beingconfigured to activate the storage cell switch transistors of thecomplementary resistance state storage cells and maintain the supplyswitch transistor of the bitcell as inactive, and to selectively directone of a first write current in a first direction through the resistancestate storage cells to write a first bit state into the bitcell, and asecond write current in a second direction opposite the first directionthrough the resistance state storage cells to write a second bit stateopposite the first bit state into the bitcell.

In Example 23 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein a first resistance stateferromagnetic device of the first resistance state storage cell is afirst magnetic-tunneling junction (MTJ) and a second resistance stateferromagnetic device of the second resistance state storage cell is asecond magnetic-tunneling junction (MTJ).

In Example 24 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein a first storage cellswitch transistor and a first MTJ of the first resistance state storagecell are series connected between the common node and a bit line signalline, a second storage cell switch transistor and a first second of thesecond resistance state storage cell are series connected between thecommon node and a bit line signal line.

In Example 25 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein said memory includes aplurality of said bitcells arranged in a rectangular array, and anorthogonal grid of supply lines including a first set of parallel spacedsupply lines coupled to the supply and arranged in a first directionwith respect to the rectangular array of bitcells, and a second set ofparallel spaced supply lines coupled to the supply and arranged in asecond direction orthogonal to the first direction of the first set ofparallel spaced supply lines wherein the supply lines are one of powersupply lines and ground supply lines.

In Example 26 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include wherein the pair ofcomplementary bit line signal lines are arranged in a first set ofparallel spaced bit line signal lines in a first direction, and thefirst and second word line signal lines are arranged in a second set ofparallel spaced word line signal lines in a second direction orthogonalto the first direction of the first set of bit line signal lines.

In Example 27 the subject matter of Examples 19-27 (excluding thepresent Example) can optionally include any of: a displaycommunicatively coupled to the processor, a network interfacecommunicatively coupled to the processor, or a battery coupled toprovide power to the system.

Example 28 is an apparatus, comprising: a plurality of signal linesincluding a pair of complementary bit line signal lines, a first wordline signal line and a second word line signal line, a supply line, anda bitcell means for storing a bit state, said bitcell means including acommon node and first and second complementary resistance state storagecells, each resistance state storage cell having an input coupled to thecommon node and an output coupled to a complementary bit line signalline, each complementary resistance state storage cell having a storagecell switch transistor having an input coupled to the first word linesignal line, and a resistance state ferromagnetic device coupled inseries with the storage cell switch transistor of the complementaryresistance state storage cell and having a resistance statecomplementary to a resistance state of the resistance stateferromagnetic device of the complementary resistance state storage cell,and a supply switch transistor means having an input coupled to thesecond word line signal line, for coupling the common node to the supplyline when activated.

In Example 29 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include a Spin Transfer Torque RandomAccess Memory (STTRAM) which includes a plurality of said bitcell meansarranged in a rectangular array, wherein a resistance stateferromagnetic device of each resistance state storage cell includes amagnetic-tunneling junction (MTJ).

In Example 30 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include a memory controller means forcontrolling read and write operations directed to the bitcell means,said memory controller means having read operation controller logicmeans for reading the bit state of the bitcell means, said readoperation controller logic means being configured to activate thestorage cell switch transistors of the complementary resistance statestorage cells and the supply switch transistor means of the bitcellmeans to cause the bitcell means to provide complementary bit statesignals on the complementary bit line signal lines wherein the bit stateof the bitcell means is read.

In Example 31 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include wherein the memory controllermeans further has write operation controller logic means for writing abit state into the bitcell means, said write operation controller logicmeans being configured to activate the storage cell switch transistorsof the complementary resistance state storage cells and maintain thesupply switch transistor means of the bitcell means as inactive todecouple the common node from the supply line, and to selectively directone of a first write current in a first direction through the resistancestate storage cells to write a first bit state into the bitcell means,and a second write current in a second direction opposite the firstdirection through the resistance state storage cells to write a secondbit state opposite the first bit state into the bitcell means.

In Example 32 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include wherein a first resistance stateferromagnetic device of the first resistance state storage cell is afirst magnetic-tunneling junction (MTJ) and a second resistance stateferromagnetic device of the second resistance state storage cell is asecond magnetic-tunneling junction (MTJ).

In Example 33 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include wherein a first storage cellswitch transistor and a first MTJ of the first resistance state storagecell are series connected between the common node and a bit line signalline, a second storage cell switch transistor and a first second of thesecond resistance state storage cell are series connected between thecommon node and a bit line signal line.

In Example 34 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include a supply which is one of a powersupply and a ground supply coupled to the supply line, a memory whichincludes a plurality of said bitcell means arranged in a rectangulararray, and an orthogonal grid of supply lines including a first set ofparallel spaced supply lines coupled to the supply and arranged in afirst direction with respect to the rectangular array of bitcell means,and a second set of parallel spaced supply lines coupled to the supplyand arranged in a second direction orthogonal to the first direction ofthe first set of parallel spaced supply lines.

In Example 35 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include wherein the pair ofcomplementary bit line signal lines are arranged in a first set ofparallel spaced bit line signal lines in a first direction, and thefirst and second word line signal lines are arranged in a second set ofparallel spaced word line signal lines in a second direction orthogonalto the first direction of the first set of bit line signal lines.

In Example 36 the subject matter of Examples 28-36 (excluding thepresent Example) can optionally include a computing system for use witha display, said computing system comprising: a memory, a processorconfigured to write data in and read data from the memory, and a videocontroller configured to display information represented by data in thememory, wherein the memory includes a magnetoresistive random accessmemory (MRAM), comprising: said pair of complementary bit line signallines, said first word line signal line and said second word line signalline, said supply line, and an array of said bitcell means, each ofwhich is an MRAM bitcell.

The described operations may be implemented as a method, apparatus orcomputer program product using standard programming and/or engineeringtechniques to produce software, firmware, hardware, or any combinationthereof. The described operations may be implemented as computer programcode maintained in a “computer readable storage medium”, where aprocessor may read and execute the code from the computer storagereadable medium. The computer readable storage medium includes at leastone of electronic circuitry, storage materials, inorganic materials,organic materials, biological materials, a casing, a housing, a coating,and hardware. A computer readable storage medium may comprise, but isnot limited to, a magnetic storage medium (e.g., hard disk drives,floppy disks, tape, etc.), optical storage (CD-ROMs, DVDs, opticaldisks, etc.), volatile and non-volatile memory devices (e.g., EEPROMs,ROMs, PROMs, RAMs, DRAMs, SRAMs, Flash Memory, firmware, programmablelogic, etc.), Solid State Devices (SSD), etc. The code implementing thedescribed operations may further be implemented in hardware logicimplemented in a hardware device (e.g., an integrated circuit chip,Programmable Gate Array (PGA), Application Specific Integrated Circuit(ASIC), etc.). Still further, the code implementing the describedoperations may be implemented in “transmission signals”, wheretransmission signals may propagate through space or through atransmission media, such as an optical fiber, copper wire, etc. Thetransmission signals in which the code or logic is encoded may furthercomprise a wireless signal, satellite transmission, radio waves,infrared signals, Bluetooth, etc. The program code embedded on acomputer readable storage medium may be transmitted as transmissionsignals from a transmitting station or computer to a receiving stationor computer. A computer readable storage medium is not comprised solelyof transmissions signals. Those skilled in the art will recognize thatmany modifications may be made to this configuration without departingfrom the scope of the present description, and that the article ofmanufacture may comprise suitable information bearing medium known inthe art. Of course, those skilled in the art will recognize that manymodifications may be made to this configuration without departing fromthe scope of the present description, and that the article ofmanufacture may comprise any tangible information bearing medium knownin the art.

In certain applications, a device in accordance with the presentdescription, may be embodied in a computer system including a videocontroller to render information to display on a monitor or otherdisplay coupled to the computer system, a device driver and a networkcontroller, such as a computer system comprising a desktop, workstation,server, mainframe, laptop, handheld computer, etc. Alternatively, thedevice embodiments may be embodied in a computing device that does notinclude, for example, a video controller, such as a switch, router,etc., or does not include a network controller, for example.

The illustrated logic of figures may show certain events occurring in acertain order. In alternative embodiments, certain operations may beperformed in a different order, modified or removed. Moreover,operations may be added to the above described logic and still conformto the described embodiments. Further, operations described herein mayoccur sequentially or certain operations may be processed in parallel.Yet further, operations may be performed by a single processing unit orby distributed processing units.

The foregoing description of various embodiments has been presented forthe purposes of illustration and description. It is not intended to beexhaustive or to limit to the precise form disclosed. Many modificationsand variations are possible in light of the above teaching.

What is claimed is:
 1. An apparatus, comprising: a plurality of signallines including a pair of complementary bit line signal lines, a firstword line signal line and a second word line signal line; a supply line;and a bitcell configured to store a bit state, said bitcell including acommon node and first and second complementary state storage cells, eachstate storage cell having an input coupled to the common node and anoutput coupled to a complementary bit line signal line, eachcomplementary state storage cell having a storage cell switch transistorhaving an input coupled to the first word line signal line, and a statestorage device coupled in series with the storage cell switch transistorof the complementary state storage cell and having a state complementaryto a state of the state storage device of the complementary statestorage cell, and a supply switch transistor having an input coupled tothe second word line signal line and configured to couple the commonnode to the supply line when activated.
 2. The apparatus of claim 1further comprising a Spin Transfer Torque Random Access Memory (STTRAM)which includes a plurality of said bitcells arranged in a rectangulararray, wherein a state storage device of each state storage cellincludes a magnetic-tunneling junction (MTJ).
 3. The apparatus of claim1, further comprising: a memory controller configured to control readand write operations directed to the bitcell, said memory controllerhaving read operation controller logic configured to read the bit stateof the bitcell, said read operation controller logic being configured toactivate the storage cell switch transistors of the complementary statestorage cells and the supply switch transistor of the bitcell to causethe bitcell to provide complementary bit state signals on thecomplementary bit line signal lines wherein the bit state of the bitcellis read.
 4. The apparatus of claim 3 wherein the memory controllerfurther has write operation controller logic configured to write a bitstate into the bitcell, said write operation controller logic beingconfigured to activate the storage cell switch transistors of thecomplementary state storage cells and maintain the supply switchtransistor of the bitcell as inactive to decouple the common node fromthe supply line, and to selectively direct one of a first write currentin a first direction through the state storage cells to write a firstbit state into the bitcell, and a second write current in a seconddirection opposite the first direction through the state storage cellsto write a second bit state opposite the first bit state into thebitcell.
 5. The apparatus of claim 1 wherein a first state storagedevice of the first state storage cell is a first magnetic-tunnelingjunction (MTJ) and a second state storage device of the second statestorage cell is a second magnetic-tunneling junction (MTJ).
 6. Theapparatus of claim 5 wherein a first storage cell switch transistor anda first MTJ of the first state storage cell are series connected betweenthe common node and a bit line signal line, a second storage cell switchtransistor and a first second of the second state storage cell areseries connected between the common node and a bit line signal line. 7.The apparatus of claim 1 further comprising a supply which is one of apower supply and a ground supply coupled to the supply line, a memorywhich includes a plurality of said bitcells arranged in a rectangulararray, and an orthogonal grid of supply lines including a first set ofparallel spaced supply lines coupled to the supply and arranged in afirst direction with respect to the rectangular array of bitcells, and asecond set of parallel spaced supply lines coupled to the supply andarranged in a second direction orthogonal to the first direction of thefirst set of parallel spaced supply lines.
 8. The apparatus of claim 1wherein the pair of complementary bit line signal lines are arranged ina first set of parallel spaced bit line signal lines in a firstdirection, and the first and second word line signal lines are arrangedin a second set of parallel spaced word line signal lines in a seconddirection orthogonal to the first direction of the first set of bit linesignal lines.
 9. A method, comprising: reading a bit state in a bitcellincluding: coupling a supply line to a common node of the bitcell todrive first and second complementary currents through first and secondcomplementary state storage cells, respectively, of the bitcell, to apair of first and second complementary bit line signal lines,respectively, of the bitcell, each state storage cell having one of afirst and second state storage device coupled between the common nodeand a bit line signal line of the first and second bit line signallines, each state storage device having a state complementary to a stateof the state storage device of the complementary state storage cell,wherein the first and second complementary currents are driven throughthe first and second state storage devices of the first and secondcomplementary state storage cells, respectively; and sensingcomplementary bit state signals on the pair of first and secondcomplementary bit line signal lines.
 10. The method of claim 9 whereineach of the first and second state storage devices of the first andsecond complementary state storage cells, respectively, includes amagnetic-tunneling junction (MTJ) and wherein a plurality of saidbitcells are arranged in a rectangular array of a Spin Transfer TorqueRandom Access Memory (STTRAM).
 11. The method of claim 9, furthercomprising: using read operation controller logic of a memory controllerto read the bit state of the bitcell, wherein the bit state readingfurther includes activating first and second storage cell switchtransistors of the first and second complementary state storage cells,respectively, to permit the first and second complementary currents topass through the first and second complementary state storage cells,respectively, and activating a supply switch transistor of the bitcellto couple the supply line to the common node of the bitcell to drive thefirst and second complementary currents through the first and secondcomplementary state storage cells, respectively, to cause the bitcell toprovide complementary bit state signals on the complementary bit linesignal lines wherein the bit state of the bitcell is read.
 12. Themethod of claim 11 further comprising using write operation controllerlogic of the memory controller to write a bit state into the bitcell,wherein the bit state writing includes: activating the first and secondstorage cell switch transistors of the complementary state storagecells; maintaining the supply switch transistor of the bitcell asinactive to decouple the supply line from the common node of thecomplementary state storage cells of the bitcell; and selectivelydirecting one of a first write current in a first direction through thestate storage cells to write a first bit state into the bitcell, and asecond write current in a second direction opposite the first directionthrough the state storage cells to write a second bit state opposite thefirst bit state into the bitcell.
 13. The method of claim 11 wherein thefirst state storage device of the first state storage cell is a firstmagnetic-tunneling junction (MTJ) and the second state storage device ofthe second state storage cell is a second magnetic-tunneling junction(MTJ).
 14. The method of claim 13 wherein driving first and secondcomplementary currents through first and second complementary statestorage cells, respectively includes driving the first complementarycurrent through the first MTJ and the first storage cell switchtransistor which are series connected between the common node and thefirst bit line signal line, and driving the second complementary currentthrough the second MTJ and the second storage cell switch transistorwhich are series connected between the common node and the second bitline signal line.
 15. The method of claim 9 wherein the bitcell is abitcell of a plurality of bitcells arranged in a rectangular array, andwherein the coupling a supply line to a common node of the bitcellfurther comprises coupling a supply line of an orthogonal grid of supplylines including a first set of parallel spaced supply lines coupled to asupply and arranged in a first direction with respect to the rectangulararray of bitcells, and a second set of parallel spaced supply linescoupled to the supply and arranged in a second direction orthogonal tothe first direction of the first set of parallel spaced supply lineswherein the supply lines are one of power supply lines and ground supplylines.
 16. The method of claim 11 wherein the sensing complementary bitstate signals on the pair of first and second complementary bit linesignal lines includes sensing the complementary bit state signals on thepair of first and second complementary bit line signal lines arranged ina first set of parallel spaced bit line signal lines in a firstdirection, and wherein the activating first and second storage cellswitch transistors of the first and second complementary state storagecells includes driving signals on first and second word line signallines arranged in a second set of parallel spaced word line signal linesin a second direction orthogonal to the first direction of the first setof bit line signal lines.
 17. A computing system, comprising: a memory;and a processor configured to cause a data write into or data read fromthe memory; wherein the memory includes a magnetoresistive random accessmemory (MRAM), comprising: a pair of complementary bit line signallines; a first word line signal line and a second word line signal line;an array of MRAM bitcells, each bitcell configured to store a bit state,each bitcell including: a common node; first and second complementarystate storage cells, each state storage cell having an input coupled tothe common node, an output coupled to a complementary bit line signalline, a storage cell switch transistor having an input coupled to thefirst word line signal line, and a state storage device coupled inseries with the storage cell switch transistor of the state storage celland having a state complementary to a state of the state storage deviceof the complementary state storage cell, wherein each state storagedevice has a polarization which in a first state is one of parallel andanti-parallel polarization and in a second state is the other ofparallel and anti-parallel polarization; a supply line which is one of apower supply line and a ground supply line; and a supply switchtransistor having an input coupled to the second word line signal lineand configured to couple the common node to the supply line whenactivated.
 18. The system of claim 17 wherein the magnetoresistiverandom access memory is a Spin Transfer Torque Random Access Memory(STTRAM) which includes a plurality of said bitcells arranged in arectangular array, wherein a state storage device of each state storagecell includes a magnetic-tunneling junction (MTJ).
 19. The system ofclaim 17, further comprising: a memory controller configured to controlread and write operations directed to the bitcell, said memorycontroller having read operation controller logic configured to read thebit state of the bitcell, said read operation controller logic beingconfigured to activate the storage cell switch transistors of thecomplementary state storage cells and the supply switch transistor ofthe bitcell to cause the bitcell to provide complementary bit statesignals on the complementary bit line signal lines wherein the bit stateof the bitcell is read.
 20. The system of claim 19 wherein the memorycontroller further has write operation controller logic configured towrite a bit state into the bitcell, said write operation controllerlogic being configured to activate the storage cell switch transistorsof the complementary state storage cells and maintain the supply switchtransistor of the bitcell as inactive, and to selectively direct one ofa first write current in a first direction through the state storagecells to write a first bit state into the bitcell, and a second writecurrent in a second direction opposite the first direction through thestate storage cells to write a second bit state opposite the first bitstate into the bitcell.
 21. The system of claim 17 wherein a first statestorage device of the first state storage cell is a firstmagnetic-tunneling junction (MTJ) and a second state storage device ofthe second state storage cell is a second magnetic-tunneling junction(MTJ).
 22. The system of claim 21 wherein a first storage cell switchtransistor and a first MTJ of the first state storage cell are seriesconnected between the common node and a bit line signal line, a secondstorage cell switch transistor and a first second of the second statestorage cell are series connected between the common node and a bit linesignal line.
 23. The system of claim 17 wherein said memory includes aplurality of said bitcells arranged in a rectangular array, and anorthogonal grid of supply lines including a first set of parallel spacedsupply lines coupled to the supply and arranged in a first directionwith respect to the rectangular array of bitcells, and a second set ofparallel spaced supply lines coupled to the supply and arranged in asecond direction orthogonal to the first direction of the first set ofparallel spaced supply lines wherein the supply lines are one of powersupply lines and ground supply lines.
 24. The system of claim 17 whereinthe pair of complementary bit line signal lines are arranged in a firstset of parallel spaced bit line signal lines in a first direction, andthe first and second word line signal lines are arranged in a second setof parallel spaced word line signal lines in a second directionorthogonal to the first direction of the first set of bit line signallines.
 25. The system of claim 17, further comprising any of: a displaycommunicatively coupled to the processor; a network interfacecommunicatively coupled to the processor; or a battery coupled toprovide power to the system.